The operating cycle of each of the cylinders of a four-stroke combustion engine is spread over two revolutions of the crankshaft. One and the same angular position of the crankshaft can therefore correspond to two different strokes in the operating cycle of the cylinder.
It is therefore important to identify where the cylinder concerned falls within the operating cycle in order to ensure that injection takes place at the correct moment for this cylinder, that is to say during an injection period that extends during the exhaust stroke in the case of indirect injection engines. If this is not done, injection will be performed out of synchronism with the cylinder operating cycle, that is to say with a phase-shift of half a cylinder operating cycle (namely one revolution of the crankshaft).
Because identifying the angular position of the crankshaft is not sufficient to identify the phases of the cycle of the cylinder concerned, it is known practice to use additional information that sets aside the uncertainty of one half-cycle over the injection period.
To do this, it is known practice to fit an angular position sensor on at least one of the camshafts. Because the camshaft performs one revolution per cycle, it then becomes possible to establish a one-to-one relationship between an angular position and a given instant in the operating cycle. However, these sensors are expensive and are tricky to fit.
It is a more particular object of the invention to provide a method that makes it possible to determine the phase of the cycle without using an angular position sensor on the camshaft.
To do this, it is known practice to run the engine while making a change to at least one first engine operating parameter (for example by increasing the injection period), and to determine the effect that this change has on the running of the engine, the change being able to bring about, in the running of the engine, effects that differ according to whether the timing is correct or out of synchronism.
However, changing the injection parameter generally leads to a change in the operation of the engine that can be discerned disagreeably by the occupants of the vehicle, such as jerky engine operation, for example, whether the timing is correct or out of synchronism.